Discussion: lecture course examination possible dates:

Thursday, 14 December (last day of lecture)

Another date in January (09 or 10 January)?

Examination will last 1 hr 30 minutes (but you may not need all of that time). Format: primarily multiple choice, a few short-answer “essay” (two or three sentences)

** Must be concluded before 15 January 2007**

Front: a narrow zone of transition between air masses of contrasting density, that is, air masses of different temperatures or different water vapor concentrations or both. ** Named by the airmass that is advancing

When 2 different air masses come together, interesting things can happen

• Fronts are actually zones of transition, but sometimes the transition zone, called a frontal zone, can be quite sharp.• The type of front depends on both the direction in which the air mass is moving and the characteristics of the air mass. • There are four types of fronts that will be described: stationary front, cold front, warm front, and occluded front.

Indentifying a front on a surface weather map or by your own weather observations

Look for: 1. Sharp temperature changes over a relatively short

distance 2. Change in moisture content 3. Rapid shifts in wind direction 4. Pressure changes 5. Clouds and precipitation patterns

Stationary front: a nearly stationary narrow zone of transition between contrasting air masses; winds blow parallel to the front but in opposite directions on the two sides of the front in the mid-latitudes, typically separates cold dense cP air from milder mP air often associated with a wide region of clouds and rain or snow on the cold side of the front.

clouds and precipitation result from overrunning, as warm humid air flows upward over the cooler air mass, more or less along the frontal surface, cools through adiabatic expansion which triggers condensation and precipitation.

Cold front: a narrow zone of transition between advancing relatively cold (dense) air and retreating relatively warm (less dense) air. • over Europe, temperature contrast across a cold front is typically greater than that across stationary or warm front. • cold frontal passage is associated with a sharp temperature drop in winter and a noticeable humidity drop in summer.

Some of the characteristics of cold fronts include the following:

• steep slope• faster movement / propogation than other fronts• most violent weather among types of fronts• move farthest while maintaining intensity• tend to be associated with cirrus well ahead of the front, strong thunderstorms along and ahead of the front, and a broad area of clouds immediately behind the front (although fast moving fronts may be mostly clear behind the front). • can be associated with squall lines (a line of strong thunderstorms parallel to and ahead of the front). • usually bring cooler weather, clearing skies, and a sharp change in wind direction.

The slope of a cold front is steeper (1:50 to 1:100) than the slope of a warm front (1:150)

Weather Feature

Before frontal passage

Region of front After frontal passage

Winds SE to SW gusty W to NW

Temperature warm sudden decrease steady cooling

Dew point high steady decreases steadily

Pressure falling steadily minimum; rapid rise steady rise

Visibility fair to poor poor then improving good

Clouds Ci, Cs, Cb Cb Cu

Precip showers heavy precip clearing

General weather characteristics of a cold front

Warm front: a narrow zone of transition between advancing relatively warm (less dense) air and retreating relatively cold (dense) air. warm front is associated with a broad cloud and precipitation shield that may extent hundred of kilometers ahead of the surface front

Some of the characteristics of warm fronts include the following: • slope of a typical warm front is more gentle than cold fronts• tend to move slowly. • are typically less violent than cold fronts. • although they can trigger thunderstorms, warm fronts are more likely to be associated with large regions of gentle ascent (stratiform clouds and light to moderate continuous rain). • are usually preceded by cirrus first (1000 km ahead), then altostratus or altocumulus (500 km ahead), then stratus and possibly fog. • behind the warm front, skies are relatively clear (but change gradually)

The type of frontal weather depends on the stability of the warmer air: when warm air is stable, a frontal inversion may exist in the upper frontal region, a steady light-to-moderate rainfall or frontal fog is observed in the presence of nimbostratus or stratus clouds, respectively. when the warm air is unstable, brief periods of heavy rainfall are observed in the presence of cumulonimbus clouds.

Weather Feature

Before frontal passage

Region of front After frontal passage

Winds NE to E variable S to SE

Temperature cool, slowly warming

steady rise warmer

Dew point steady rise steady increases, then steady

Pressure usually falling levels off slight rise, followed by fall

Visibility poor improving fair

Clouds Ci, Cs, As, Ns, St, fog

stratus Clearing with scattered Sc

Precip light to moderate, can be SN or RA

drizzle or nothing usually none

General weather characteristics of a warm front

Occluded front (occlusion): a narrow zone of transition formed when a cold front overtakes a warm front.

Dry Line:• boundary that separates moist air mass from a dry air mass• also called “Dew Point Front”• most commonly found just east of the Rocky Mountains; rare east of the Mississippi River• common in TX, NM, OK, KS, and NE in spring and summer

Rocky MountainsDry Line

Hot, dry air

Gusty southwest winds

Warm, moist air

Southeast winds

States like Texas, New Mexico, Oklahoma, Kansas, and Nebraska frequently experience dry lines in the spring and summer. Dry lines are extremely rare east of the Mississippi River.

How do fronts form?

Terms 1, 5, 9: Diabatic Terms

Terms 2, 3, 6, 7: Horizontal Deformation Terms

Terms 10 and 11: Vertical Deformation Terms

Terms 4 and 8: Tilting Terms

Term 12: Vertical Divergence Terms

Bluestein (Synoptic-Dynamic Met. In Mid-Latitudes, vol. II, 1993)

1 2 3 4

5 6 7 8

9 10 11 12

Three-Dimensional Frontogenesis Equation

Assumptions to Simplify the Three-Dimensional Frontogenesis Equation

θ

θ + 1

θ + 2

y’

x’

• y’ axis is set normal to the frontal zone, with y’ increasing towards the cold air (note: y’ might not always be normal to the isentropes)

• x’ axis is parallel to the frontal zone

• Neglect vertical and horizontal diffusion effects

Fd

dt y

u

y x

v

y y

w

y z y

d

d t

Simplified Form of the Frontogenesis Equation

A B C D

Term A: Shear term

Term B: Confluence term

Term C: Tilting term

Term D: Diabatic Heating/Cooling term

Frontogenesis: Shear Term

Shearing Advection changes orientation of isotherms

Carlson, 1991 Mid-Latitude Weather Systems

Frontogenesis: Confluence Term

Cold advection to the north

Warm advection to the south

Carlson, 1991 Mid-Latitude Weather Systems

Carlson (Mid-latitude Weather Systems, 1991)

Why are cold fronts typically stronger than warm fronts? Look at the shear and confluence terms near cold and warm fronts

Shear and confluence terms oppose one another near warm fronts

Shear and confluence terms tend to work together near cold fronts

Frontogenesis: Tilting Term

Adiabatic cooling to north and warming to south increases horizontal thermal gradient

Carlson, 1991 Mid-Latitude Weather Systems

Frontogenesis: Diabatic Heating/Cooling Term

frontogenesis

frontolysis

T constant T increases

T increases T constant

Carlson, 1991 Mid-Latitude Weather Systems

Petterssen (1968)

Frontogenesis/Frontolysis with Deformation with No Diabatic Effects or Tilting Effects

Fd

dtD ef D ivR

1

22co s

D efv

x

u

y

u

x

v

yR

2 21

2

ß= angle between the isentropes and the axis of dilatation

where:

and

MID-LATITUDE CYCLONES

the cause of most of the stormy weather in the northern hemisphere, especially during the winter season

Understanding their structure and evolution is crucial for predicting significant weather phenomena such as blizzards, flooding rains, and severe weather.

Mid-latitude (or frontal) cyclones large traveling atmospheric cyclonic storms up to 2000 kilometers in diameter with centers of low atmospheric pressure located between 30 degrees and 60 degrees latitude (since the continental United States is located in this latitude belt, these cyclones impact the weather in the U.S.) form along the polar front an intense system may have a surface pressure as low as 970 millibars normally, individual frontal cyclones exist for about 3 to 10 days moving in a generally west to east direction precise movement of this weather system is controlled by the orientation of the polar jet stream in the upper troposphere commonly travels about 1200 kilometers in one day

How many mid-latitude cyclones can you identify from this satellite image?

How many mid-latitude cyclones can you identify from this satellite image?

What causes mid-latitude cyclones to form?

Surface extra-tropical (i.e., non-hurricane) cyclones are directly coupled with the upper-levels.

Typically an upper-level trough, and its associated super-geostrophic wind maximum, move over a surface temperature gradient.

Remember our equation for relative vorticity (spin) generation? Notice the 2nd term: “baroclinic term”. The stationary front / temperature boundary provides the necessary baroclinic energy for the surface cyclone to develop.

Upper-air surface

Stages of mid-latitude cyclone development:Two models of development

Mid-latitude cyclone model application: 6 June 1944(Petterssen’s forecast)

• Mid-latitude cyclones are "deep" pressure systems extending from the surface to the tropopause• A surface low-pressure system grows if there is vertical wind shear (winds increasing with height) and thermal instability (convection). • The factors that lead to lowering of the pressure at the surface are:

• Diverging airflow at high altitudes• Inflow of warm, moist air at low and mid levels. • Latent heat release caused by convection in the warm air mass sector of the growing storm

system.